1. Field of the Invention
The present invention relates to an acceleration sensor, and more particularly to an electrostatic capacitance type acceleration sensor.
2. Description of the Background Art
As one of the principles of a conventional acceleration sensor for detecting acceleration in the substrate thickness direction, there is a method for detecting a change in electrostatic capacitance in accordance with the acceleration. As an acceleration sensor based on this method, there is proposed in Japanese Patent Laid-Open No. 05-133976 (page 16, FIGS. 23 and 24) an acceleration sensor including, for example, a torsion beam, an inertia mass body, a detection frame, and a detection electrode as main components.
The acceleration sensor includes one detection frame having a surface facing a substrate. The inertia mass body is provided on one end part of the detection frame. Further, the detection frame is supported on the substrate so as to be rotatable with the torsion beam as the axis of rotation. Further, the detection electrode for detecting this rotational displacement is provided under the detection frame.
When acceleration in the substrate thickness direction is applied to the acceleration sensor constituted as described above, inertia force in the substrate thickness direction acts on the inertia mass body. Since the inertia mass body is provided on the one end part, that is, at a position deviated from the axis of rotation in the substrate in-plane direction, this inertia force acts on the detection frame as a torque around the torsion beam. As a result, the detection frame is rotationally displaced.
The distance between the detection frame and the detection electrode is changed by this rotational displacement, which causes electrostatic capacitance formed by the detection frame and the detection electrode to be changed. The acceleration is measured from this change in electrostatic capacitance.
Gravity always acts downward on the inertia mass body. Thereby, the inertia mass body is in the state of being displaced downward from the axis of rotation of the detection frame.
When acceleration in the substrate in-plane direction and in a direction crossing the axis of rotation is applied to the acceleration sensor in this state, the point where the inertia force acts on the detection frame is positioned lower than the axis of rotation. Further, this inertia force has a component orthogonal to the axis of rotation. As a result, the detection frame is rotationally displaced by receiving torque around the axis of rotation. That is, also when acceleration along an axis other than the axis as the object to be detected by the acceleration sensor is applied, the detection frame is rotationally displaced.
Further, also when angular acceleration around the torsion beam is applied to the acceleration sensor, the detection frame is rotated by the inertia force applied to the inertia mass body.
Further, also when angular velocity is applied to the acceleration sensor, the detection frame may be rotated under the influence of centrifugal force applied to the inertia mass body.
In the above described conventional acceleration sensor, the rotation of the detection frame due to acceleration of other axis, angular acceleration, and angular velocity cannot be distinguished from the rotation of the detection frame due to acceleration in the substrate thickness direction which is the object to be detected. Therefore, there is a problem that the detection error of acceleration is increased.